These are two excellent books. Chess Fundamentals was published first, but A Primer of Chess is targeted more towards the beginner. There is considerable overlap of content, because Capablanca's purpose is to convey principles and guidelines and simple endgame situations more than anything else.
Chess Fundamentals is the first chess book that I felt actually taught me anything I could understand (before that, I'd mainly played over game collections) and immediately apply. If I had both books to choose from, and I knew someone just starting out in chess, or who has not really made a close study of the elements, I'd recommend A Primer of Chess ... but I'd then recommend that Chess Fundamentals be read next, because it can act as summary, review, and reference of a sort.
As to your question about whether there is much point in reading them both at the same time....
NEVER read more than one chess book at a time. In fact, don't read these books, digest them. These aren't novels, and though they are not particularly deep as far as intellectual content, mastering the subject matter requires repetition and active practice. Don't hurry through these books, either; don't move on from one topic to the next before you've absolutely mastered that former topic. Set up each position from every diagram against a chess engine playing at full strength; if you cannot achieve the required result (win or draw), you have not mastered the topic. If you cannot force mate with K+Q vs K with less than 30 seconds on the clock against Rybka/Houdini/Stockfish/Fritz/whatever, you have not mastered that checkmate.
Does any of you can compare two classic books by Capablanca:
- Chess fundamentals
- A Primer of Chess
to each other. It there much point to read them contemporarily?